Rahil Valani

Superwalking droplets: active particles with a memory

Active particles come in many forms, from bacteria, cells and algae to synthetic colloids and microrobots, all converting energy into directed motion. Their dynamics become particularly interesting when their motion modifies the surrounding environment, which in turn acts back on them. What happens when this feedback gives an active particle a memory of its past? In this talk, I will explore this question using superwalking droplets which are millimetre-scale liquid droplets that self-propel across a vertically vibrated fluid bath through interaction with the waves they generate. The feedback between droplet and wave gives these active wave–particle entities an effective memory and produces surprisingly rich dynamics. I will discuss how this memory shapes individual locomotion, including steady walking and stop-and-go motion, before turning to wave-mediated interactions, collective behaviour, and emergent clusters formed by very large superwalkers. Finally, I will show how reduced models of memory-driven wave–particle motion lead to low-dimensional dynamical systems related to the Lorenz equations, providing a bridge between fluid mechanics, active matter and nonlinear dynamics.